Passive entry systems are used to enable a user to remotely access or perform some operation to a vehicle. For example, a vehicle may be equipped with a remote entry system that includes a controller located on the vehicle, which wirelessly communicates with an electronic smart key possessing the ability to transmit and receive information on different frequencies or in a full-duplex or half-duplex mode using the same frequency. Such a 2-way capable device may be referred to as “key fob.” The key fob may include controls that allow the user to request access to the vehicle or start the engine. In some applications, a user may engage a door handle of the vehicle, triggering the vehicle controller to transmit a challenge signal to the key fob for authentication. In response, the key fob generates a response signal and transmits the response to be received by the controller. Upon receiving the signal, the controller performs the requested action, such as unlocking the vehicle doors. A challenge-response authentication process using the key fob may also apply to starting a vehicle engine.